The subject matter disclosed herein relates to improvements in valve technology with particular discussion about embodiments of a modulating device that generates an output that relates to displacement of components in a valve assembly.
Valve assemblies like control valves can regulate flow of a process fluid in a process line. In their broadest configuration, these valve assemblies have a valve component with a plug (or moving element) that moves relative to a seat (or stationary element). This configuration also includes an actuator component that utilizes pneumatic signals (or flow of working fluid) to change the position of the plug. The actuator component can have a diaphragm (or actuating element) and a stem element that couples the diaphragm with the plug. During operation, the pneumatic signals cause movement of the diaphragm to set the position of the plug, often in accordance with signals that originate from a process control system. These signals instruct operation of the valve assembly to define an amount of process fluid that flows through the valve assembly and, thus, maintain operating conditions across the process line.
Operating conditions in many processes are sensitive to even small deviations in the amount of process fluid that flows through the valve assembly. This feature warrants construction of the valve assembly in a manner that can accurately and repeatability position the plug relative to the seat. Often, the valve assembly incorporates a valve positioner and a sensor, or like device, that measures the relative movement of the actuator (or its constituent components). This sensor provides feedback to the valve positioner about displacement of the plug relative to the seat. The valve positioner can use this feedback to regulate the pneumatic signals, which in turn operates the actuator to position the plug relative to the seat to within some reasonable operating tolerances or thresholds.
Most facilities with process lines allocate only a finite amount of power for operation of the valve assembly. This power constraint can influence design choices, namely for sensors and other electronics that are found on the valve assembly. For example, designs for the valve assembly may incorporate sensors (e.g., Hall Effect sensors) that consume less power than other sensors. These low power sensors often have an operating range (or detecting range) that is narrow relative to these other sensors that consume more power.
Unfortunately, the position of the plug relative to the seat may require movement of the actuator that exceeds the operating range of these low power sensors. Conventional designs for a valve assembly address this problem with a motion converter that reduces the relative movement of the actuator to motion that “fits” within the operating range of the sensor. Examples of the motion converter include linkages, lever arms, and/or cam-follower mechanisms, each of which can convert linear movement of the actuator to rotary displacement that fits within the operating range of the sensor. However, these mechanisms add cost and complexity to the valve assembly and, in many cases, are susceptible to defects in operation, wear, and damage due to dirt (and debris) and vibrations that prevail in and around the process line.